Several different processes have been used for the oxidation of cyclohexane into a product mixture containing cyclohexanone and cyclohexanol. Such product mixture is commonly referred to as a KA (ketone/alcohol) mixture. The KA mixture can be readily oxidized to produce adipic acid, which is an important reactant in processes for preparing certain condensation polymers, notably polyamides. Given the large quantities of adipic acid consumed in these and other processes, there is a need for cost-effective processes for producing adipic acid and its precursors.
One technique presently used for cyclohexane oxidation employs metaboric acid as a catalyst. Although metaboric acid is a somewhat effective oxidation catalyst, certain drawbacks are associated with its use. A principal drawback is the need for catalyst recovery, which typically involves hydrolysis of the reaction mixture, aqueous and organic phase separation, and dehydration of boric acid. These steps introduce considerable complexity and expense into the overall process.
Organic cobalt salts, such as cobalt octanoate, have been widely used for oxidizing cyclohexane into KA mixtures. Various homogenous metal catalysts also have been proposed for oxidizing cycloalkanes, such as salts of chromium, iron, and manganese, with varying results in terms of cyclohexane conversion and ketone/alcohol selectivities.
Two-stage processes also have been used for cycloalkane oxidation. In a first stage of one typical two-stage process, cyclohexane is oxidized to form a reaction mixture containing cyclohexyl hydroperoxide (CHHP). In a second stage, CHHP is decomposed, with or without use of a catalyst, to form a KA mixture. An example of a two-stage process is described in U.S. Pat. No. 6,284,927 to Druliner et al., in which an alkyl or aromatic hydroperoxide is oxidized in the presence of a heterogeneous catalyst of Au, Ag, Cu or a sol-gel compound containing particular combinations of Fe, Ni, Cr, Co, Zr, Ta, Si, Mg, Nb, Al and Ti, wherein certain of these metals are combined with an oxide. Other catalysts that have been proposed for the second stage of two-stage oxidation processes include salts of manganese, iron, cobalt, nickel, and copper.
WO 00/53550 and companion U.S. Pat. No. 6,160,183 to Druliner et al. describe a heterogeneous catalyst for so-called direct oxidation of cycloalkanes to form a KA mixture. The catalysts described include gold, gold sol-gel compounds, and sol-gel compounds containing particular combinations of Cr, Co, Zr, Ta, Si, Mg, Nb, Al and Ti, wherein certain of these metals are combined with an oxide.
Fan et al., “Environmentally Benign Oxidations of Cyclohexane and Alkenes with Air Over Zeolite-encapsulated Au Catalysts,” Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, discloses catalysts for the oxidation of cyclohexane and alkenes. Au/NaY is said to yield high turnover frequency and product selectivities when used as an oxidation catalyst for cyclohexane.
There remains a need for cost-effective methods for oxidizing cycloalkanes to KA mixtures, particularly methods employing catalysts that yield high cycloalkane conversions, high ketone and alcohol selectivities, and relatively low cycloalkyl hydroperoxide concentrations.